Abstract. During the CalNex study (15 May to 16 June 2010) a large suite of
instruments was operated at the Los Angeles area ground supersite to
characterize the sources and atmospheric processing of atmospheric
pollution. The thermal-desorption proton-transfer-reaction mass-spectrometer
(TD-PTR-MS) was deployed to an urban area for the first time and detected
691 organic ions in aerosol samples, the mean total concentration of which
was estimated as 3.3 μg m−3. Based on comparison to total organic
aerosol (OA) measurements, we estimate that approximately 50% of the OA
mass at this site was directly measured by the TD-PTR-MS. Based on
correlations with aerosol mass spectrometer (AMS) OA components, the ions
were grouped to represent hydrocarbon-like OA (HOA), local OA (LOA),
semi-volatile oxygenated OA (SV-OOA), and low volatility oxygenated OA
(LV-OOA). Mass spectra and thermograms of the ion groups are mostly
consistent with the assumed sources and/or photochemical origin of the OA
components. The mass spectra of ions representing the primary components HOA
and LOA included the highest m/z, consistent with their higher resistance to
thermal decomposition, and they were volatilized at lower
temperatures (~ 150 °C). Photochemical ageing
weakens C-C bond strengths (also resulting in chemical fragmentation), and
produces species of lower volatility (through the addition of functional
groups). Accordingly the mass spectra of ions representing the oxidized OA
components (SV-OOA, and LV-OOA) lack the highest masses and they are
volatilized at higher temperatures (250–300 °C). Chemical
parameters like mean carbon number (nC), mean carbon
oxidation state (OSC), and the atomic ratios O / C and H / C
of the ion groups are consistent with the expected sources and photochemical
processing of the aerosol components. Our data suggest that chemical
fragmentation gains importance over functionalization as photochemical age
of OA increases. Surprisingly, the photochemical age of OA decreases during
the daytime hours, demonstrating the importance of rapid production of new
(photochemically young) SV-OOA during daytime. The PTR detects higher
organic N concentrations than the AMS, the reasons for which are not well
understood and cannot be explained by known artifacts related to PTR or the
AMS. The median atomic N / C ratio (6.4%) of the ion group representing
LV-OOA is a factor 2 higher than N / C of any other ion group. This suggests a
multiphase chemical source involving ammonium ions is contributing to
LV-OOA.